It has been well known that in a conventional image reading device, an object, such as document, is read by irradiating the document with light from a light source and receiving the light reflected from the document at light receiving elements.
In such a conventional image reading device, a document placed on a document placing table is read by a reading unit having light sources and light receiving elements. The light receiving elements are aligned in a main scanning direction. To read the document, the reading unit is moved in an auxiliary scanning direction perpendicular to the main scanning direction.
The conventional image reading device has a function for detecting the size of a document placed on a document placing table. More specifically, a top end portion of the document is read and the side edge of the document is extracted to determinate the size of the document.
Alternatively, the size of a document can be determined through a pre-scan performed prior to reading a document. In the pre-scan, the reading unit is moved to scan over the entire area of the document.
Generally, in order to adjust a light amount emanated from a light source, the reading unit reads a white reference member prior to reading the document. The light amount is adjusted so that the output value of the reading unit is not saturated when a white color (the white reference member) is read.
The white reference member is irradiated with light having an adjusted intensity in order to define white data. Black data is defined by the output from the reading unit when the white reference member is not irradiated with the light. When a document is read, the document is read with the light having the adjusted intensity. Shading compensation is performed with respect to the white data and the black data. On the shading compensation, a pixel value is determined by correcting the output value for the reading unit. A process for adjusting the light amount and a process for generating the white data and the black data will be hereinafter referred to as a “pre-process”.
Further, in an image reading device performing a pre-scan, prior to the pre-scan, the pre-process is performed for the sake of the subsequently performed pre-scan. A document is pre-scanned according to the results of the pre-process. Subsequent to the pre-scan, the reading unit is once returned to the home position. A main-scan is performed after the pre-process for the main-scan is performed.
If the pre-process is performed for each of the pre-scan and the main-scan each time by reading the white reference member, it takes longer to read a single page of a document due to duration of time required for the pre-process.